1. Field of the Invention
The present invention relates to a charge transfer device in which two-phase driving pulses are applied to a number of two-layered transfer electrodes arranged above a transfer channel to perform a transfer operation, and a solid-state image pickup device using the same.
2. Description of the related Art
Generally, in a solid-state image pickup device such as CCD (Charge Coupled Device) area sensor or the like, signal charges which are photoelectrically converted by a photosensor serving as an image receiving element are transferred in a vertical direction by plural vertical transfer registers, and the signal charge thus transferred by each of the vertical transfer registers is transferred-in a horizontal direction by a horizontal transfer register which is driven in two-phase.
FIGS. 5A to 5C show the construction of a horizontal transfer register and an output portion in a conventional solid-state image pickup device (CCD area sensor), wherein FIG. 5A is a cross-sectional view showing the arrangement of electrodes of the horizontal transfer register, FIG. 5B shows the potential distribution corresponding the electrode arrangement, and FIG. 5C is a plan view showing the structure in the neighborhood of the last stage of the horizontal transfer register.
In FIG. 5, a number of two-layered transfer electrodes 51 are arranged along a charge transfer direction X above a transfer channel 50, and a gate electrode 52 is formed so as to be adjacent to a transfer electrode 51L located at the last stage thereof (hereinafter referred to as “last-stage transfer electrode”). Each of the two-layered transfer electrodes 51, 51L is constructed by a transfer electrode 51a, 51La as the first layer and a transfer electrode 51b, 51Lb as the second layer. Further, a two-phase driving pulse øH1, øH2 is applied to the two-layered transfer electrode 51 (containing 51L), and a gate voltage (DC voltage) VHOG is applied to the gate electrode 52.
Besides, in the potential distribution of the transfer channel 50, the potential level corresponding to the transfer electrode 51a, 51La of the first layer is set to be deeper the potential level corresponding to the transfer electrode 51b, 51Lb of the second layer by doping an area below the transfer electrode 51b, 51Lb of the second layer with such impurities as to shallow the potential level. With the doping of these impurities, in the area of the transfer channel 50, a storage portion (1) is formed below the transfer electrodes 51a, 51La of the first layer and a transfer portion (2) is formed below the transfer electrodes 51b, 51Lb of the second layer.
Further, a floating diffusion area (hereinafter referred to as “FD area”) is connected through the gate electrode 52 to the last-stage transfer electrode 51L. The FD area 53 serves to detect the charge amount of signal charges transferred by the last-stage transfer electrode 51L and convert the charges to the voltage corresponding to the charge amount thus detected.
Next, the manufacturing process of the horizontal transfer register in the conventional solid-stage image pickup device will be described with reference to FIGS. 6A to 6D.
First, as shown in FIG. 6A, an N-type transfer channel is formed on a semiconductor substrate, and then the electrodes 51a, 51La of the first layer are formed. Subsequently, as shown in FIG. 6B, a predetermined portion is covered by a resist mask 54, and then P-type impurities to shallow the channel potential are doped by an ion implantation method or the like with the electrodes 5151a, 51La of the first layer as a mask.
Subsequently, as shown in FIG. 6C, the peripheral portions of the electrodes 51a, 51La of the first layer are covered by insulating material by oxidizing the electrodes 51a, 51La of the first layer or the like, and then the electrodes 51b, 51Lb, 52 of the second layer are formed. Finally, as shown in FIG. 5D, the electrode 52 at the end position is wired to form a gate electrode. Further, the other electrodes 51a, 51La of the first layer and the electrodes 51b, 51Lb of the second layer which are adjacent to each other are connected to each other to form the transfer electrodes 51, 51L of the second layer.
The maximum operating charge amount in the horizontal transfer register (hereinafter merely referred to as “operating charge amount”) increases in proportion to the potential difference between the storage portion (1) and the transfer portion (2) and the electrode length Lst and the channel width W of the storage portion (1). In other words, the operating charge amount of the horizontal transfer register is determined by two parameters of the potential difference between the storage portion (1) and the transfer portion (2) and the electrode area of the storage portion (1) (the effective electrode area corresponding to the channel width).
The FD area 53 is formed of a more minute area than the transfer electrodes 51, 51L in order to increase the charge-to-voltage conversion gain and thus enhance the detection sensitivity. Therefore, the transfer channel 50 of the horizontal transfer register is designed so that the channel width W is reduced from the vicinity of the last-stage transfer electrode 51L to the FD area 53.
In this case, the electrode area of the storage portion of the last-stage transfer electrode 51L is reduced as the channel width W is reduced, so that the operating charge amount is reduced by the amount corresponding to the reduction of the electrode area. Therefore, it may be considered to alter the two parameters in order to prevent the reduction of the operating charge amount. However, with respect to the potential difference between the storage portion (1) and the transfer portion (2), if the potential difference is increased to ensure the operating charge amount, the amplitude at the transfer operation time is increased and thus the consumption power is increased. Therefore, in the present situation, there may be considered a method of increasing the electrode length Lst of the storage portion (1) of the last-stage transfer electrode 51L as shown in FIG. 7 to increase the electrode area at that place and ensure the operating charge amount.
However, if the electrode length Lst of the storage portion (1) of the last-stage transfer electrode 51L is increased as described above, the transfer distance (Lst+Ltr) at the last-stage transfer electrode 51L is also increased by the amount corresponding to the increase of the electrode length Lst, and thus the signal charge is hard to flow and thus the transfer efficiency is lowered. As a result, a transfer failure of the signal charge at the last-stage transfer electrode 51L occurs (the transfer is left uncompleted or the like), and image quality deterioration (color mixture, image tear or the like) occurs.